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An investigation was performed to increase the heat transfer rates from methane flames while simultaneously minimizing the soot and NOx emissions at extensive firing rates. The partially premixed flames responded favorably to the strain rate and the increase in temperature-soot interactions across the two reaction zones. The pulsation of partially premixed flames increased the temperature fluctuations by 27.3% and the blowout limit by 35.4%. The premixed flame wrinkling increased the radiation heat transfer rates by 38%, while the convection heat transfer rates increased by about 5 times. The overall heat transfer rate thus increased by 18.5% at an optimum Strouhal number, St = 0.32. The flame was shortened by 48%, while the NOx emission index decreased to 0.17 g/kg fuel. Inverse flames with under-ventilation increased the temperature-soot interaction. Shifting the peak turbulent kinetic energy into the lean side in normal flames increased both the radiation and convection heat transfer rates respectively by 13 and 9%. On the other hand, shifting the peak turbulent kinetic energy into the rich side reduced the NOx emissions. Exciting two concentric streams increased the turbulent kinetic energy and provided an innovative control of the pulsation effects. The soot growth and oxidation rates were thus controlled by setting the phase shift between the two streams at 20° and the corresponding frequency ratio at 4.0 such that the total heat transfer rate increased by 40.7%.

Single and Double Flow Pulsations of Normal and Inverse Partially Premixed Methane-Air Flames